Eval buffer should evaluate the unsaved current buffer and not the file on disk

[xaderfos]

At the moment eval-buffer runs the assembler against the file on disk, any unsaved changes are not included.

Ideally we should either be able to pipe the buffer to stdin or at least create a tmp file to store the modified buffer and pass that to the assembler.

Acceptance criteria:

• uxntal-eval-buffer evaluates the contents of the buffer and not the contents of the file on disk